All Pilots are taught to be aware
of wake turbulence. However, recent incidents indicate that pilots need
to keep in mind how severe wake turbulence can be. In any event, wake
turbulence is still out there and it can put a pilot and the aircraft
at risk. This page was prepared as a reminder to pilots, to make them
aware of wake turbulence and how to best avoid it.

Remember, the best defense against wake turbulence is to know and avoid areas where it occurs.

What is Wake Turbulence

All Aircraft produce wake
turbulence. Wake vortices are formed any time an airfoil is producing
lift. Lift is generated by the creation of a pressure differential over
the wing surfaces. The lowest pressure occurs over the upper surface
and the highest pressure under the wing. This pressure differential
triggers the rollup of the airflow aft of the wing resulting in
swirling air masses trailing downstream of the wingtips. Viewed from
behind the generating aircraft, the left vortex rotates clockwise and
the right vortex rotates counterclockwise.

The
intensity or strength of the vortex is primarily a function of aircraft
weight and configuration (flap setting etc.). The strongest vortices
are produced by heavy aircraft, flying slowly, in a clean
configuration. For example, a large or heavy aircraft that must reduce
its speed to 250 knots below 10,000 feet and is flying in a clean
configuration while descending, produces very strong wake. Extra
caution is needed when flying below and behind such aircraft.

Induced Roll - The Greatest Hazard

While there have been rare
instances where wake turbulence caused structural damage, the greatest
hazard is induced roll and yaw. This is especially dangerous during
takeoff and landing when there is little altitude for recovery.

Short wing span aircraft are most
susceptible to wake turbulence. The wake turbulence-induced roll rates
can be extreme. Countering roll rates may be difficult or impossible
even in a high performance aircraft with excellent roll control
authority.

Parallel or crossing Runways - Stay Heads Up for the Wake

During takeoff and landing, the vortices sink toward the ground and
move laterally away from the runway when the wind is calm. A 3 to 5
knot crosswind
will tend to keep the upwind vortex in the runway area and may cause the downwind vortex to drift toward another runway.

At altitude, vortices sink at a rate of 300
to 500 feet per minute and stabilize about 500 to 900 feet below the
flight level of the generating aircraft.

Helicopter Wake

Helicopters
also produce wake turbulence.
Helicopter wakes may be of significantly greater strength than those
from a fixed wing aircraft of the same weight. The strongest wake can
occur when the helicopter is operating at lower speeds (20 - 50 knots).
Some mid-size or executive class helicopters produce wake as strong as
that of heavier helicopters This is because two blade main rotor
systems, typical of lighter helicopters, produce stronger wake than
rotor systems with more blades.

Stay On or Above Leader's Glide Path

Incident data shows that the
greatest potential for a wake vortex incident occurs when a light
aircraft is turning from base to final behind a heavy aircraft flying a
straight-in approach.
Use extreme caution to intercept final above or well behind the heavier
aircraft. When a visual approach is issued and accepted to visually
follow a preceding aircraft, the pilot is required to establish a safe
landing interval behind the aircraft s/he was instructed to follow. The
pilot is responsible for wake turbulence separation. Pilots must not
decrease the separation that existed when the visual approach was
issued unless they can remain on or above the flight path of the
preceding aircraft.

Warning Signs

Any uncommanded aircraft movements
(i.e., wing rocking) may be caused by wake. This is why maintaining
situational awareness is so critical. Ordinary turbulence is not
unusual, particularly in the approach phase. A pilot who suspects wake
turbulence is affecting his or her aircraft should get away from the
wake, execute a missed approach or go-around and be prepared for a
stronger wake encounter. The onset of wake can be insidious and even
surprisingly gentle. There have been serious accidents where pilots
have attempted to salvage a landing after encountering moderate wake
only to encounter severe wake vortices. Pilots should not depend on any
aerodynamic warning, but if the onset of wake is occurring, immediate
evasive action is a MUST!

How to Avoid Wake Turbulence

Takeoff

If you think wake turbulence from the preceding
aircraft may be a factor, wait at least 2 or 3 minutes before taking
off. (See AIM para. 7-58 b & c). Before taking the runway, tell the
tower that you want to wait. Plan your takeoff to liftoff before the
rotation point of the preceding aircraft.

Climb

If you can, climb
above the preceding aircraft's flight path. If you can't out climb it,
deviate slightly upwind, and climb parallel to the preceding aircraft's
course. Avoid headings that cause you to cross behind and below the
preceding aircraft.

Crossing

If you must cross
behind the preceding aircraft, try to cross above its flight path or
(terrain permitting) at least 1,000 feet below.

Following

Stay either on or above the preceding aircraft's flight path, upwind, or at least 1,000 feet below.

Approach

Maintain a position on or above the preceding aircraft's flight path with adequate lateral separation.

Landing

Ensure that your
touchdown point is beyond the preceding aircraft's touchdown point, or
land well before a departing aircraft's rotation point.

Crossing Approaches

When landing behind another aircraft on crossing approaches, cross above the other aircraft's flight path.

Crosswinds

Remember crosswinds may affect the position of the vortices. Adjust takeoff and landing points accordingly.

Helicopters

Helicopter wake
vortices may be of significantly greater strength than fixed wing
aircraft of the same weight. Avoid flying beneath the flight paths of
helicopters.

Tell Your Story to ASRS

You can help support an FAA effort to make a safe system even safer.

If you have recently experienced wake turbulence, tell the Aviation Safe Reporting System (ASRS)
about it. The ASRS is one of the ways the industry learns about safety
issues. You can also help support an FAA effort to reduce the frequency
and danger of wake vortex events.

Report participation is voluntary and all
identifying information will be removed before the ASRS research data
are given to the FAA. You can report a wake turbulence incident to the
ASRS by completing a NASA/ASRS form obtained from your company, a
Flight Service Station, or directly from NASA. Write to NASA/ASRS at
625 Ellis Street, Suite 305, Mountain View, California 94043, phone
(415) 969-3969 or fax (415) 967-4170.

The people at ASRS may call you and request
to interview you about the wake vortex incident. Participation in the
callback program is also voluntary, and confidential Your information
will help improve the safe of the National Airspace System.

"It felt as though we had hit a twenty-foot thick concrete wall."Statement from a corporate pilot after hitting the wakefrom a large aircraft in a slow descent across his flight path.

"We were cleared for a visual approach to the right runway and to
maintain visual separation on a wide-body cleared to land on the left.
Our yaw damper was MEL'ed inoperative and the aircraft kind of wallowed
in the landing configuration. At 2,200 feet MSL. on about a 7 mile
final, the aircraft began an abrupt roll to the left and the nose
pitched down. Full scale deflection of the yoke to the right did not
arrest the left roll and for a moment, it appeared the aircraft was
going to roll over onto its back. Suddenly, the aircraft began to
recover as the ailerons regained effectiveness. The rest of the
approach and landing were normal. The winds were from the left and must
have caused the wake vortices from the wide-body to drift toward the
right runway final."Statement from pilot flying a regional turboprop.

"While holding for an IFR release, a large military transport was
cleared for a low approach and to remain in the pattern. Approximately
2 to 3 minutes later, I was cleared for takeoff. After takeoff, I
cleaned the aircraft up and climbed about 20 knots faster than normal.
Going through 1,200 feet MSL, I lowered the nose slightly because I
thought I had cleared the crosswind path of the military transport.
About that time, the aircraft began a smooth roll to the left and felt
like it was trying to pitch up. I applied nose down pressure and then
heard the engines cavitate and we pitched down rather abruptly. The
aircraft then felt solid, so I throttled back and gradually pulled up
and resumed the climb. The incident shook up some of the passengers, so
I explained to them what I thought had happened and that I was going to
return to the airport. In all my years of flying, this is the first
time I have encountered the wake from a large aircraft."Statement from a light transport pilot.

The above are actual excerpts from reports to the Aviation Safety Report System (ASRS).

For Additional Information

Order the free Advisory Circular, AC-90-23E from:

DOT, M-443.2

General Services Section

Washington, D.C. 20590

A self addressed mailing label will speed processing and delivery

A wake turbulence Training Aid has been
developed by the FAA and industry. The training aid includes a report,
a videotape, and a CD-ROM. Copies of the training aid are available for
a fee from:

National Technical Information Service

U.S. Department of Commerce

5285 Port Royal Road

Springfield, VA 22161

or call: 703.487.4650. Request the following NTIS Accession Numbers:

Report PB95780136

Videotape AVA19661-VNB1

CD-ROM PB95502613

Also, there is a section on wake turbulence in the current Aeronautical Information Manual (AIM).

To receive a printed copy of publication No: ASY-20 95/003
"CAUTION Wake Turbulence" brochure from which these pages are based, write:

FAA/ASY-20

400 7th Street, S.W.

Washington, D.C. 20590

You may also leave your request with an automated telephone answering service by dialing: (202) 267-7770